In borehole acoustic logging techniques, acoustic signals are generated by a source in the fluid filled borehole, propagate through various paths including the formation and borehole fluid, and are detected at a receiver in the borehole spaced from the source. The source and receiver are typically suspended from a wireline cable or coil tubing. Such techniques can be used for characterizing the formation around the borehole or for imaging the surrounding structures. In examples such as these, tube waves (Stoneley waves) which travel along the borehole are often the most energetic part of the received waveform and can interfere with signals of interest such as flexural mode signals or reflected formation compressional waves.
Various methods have been proposed previously to attenuate tube waves passing along the borehole from the source to the receiver and so reduce the interfering effect. Such methods rely on placing an attenuator in the borehole between the source and receiver. One form of attenuator which has been proposed for use in borehole seismic systems is one or more inflatable bladders which are positioned in the borehole such as is described in U.S. Pat. No. 4,858,718 or U.S. Pat. No. 5,005,666. The bladders are inflated with gas to fill the borehole and provide acoustic isolation between the source and receiver. The bladders are provided with a supply of pressurized gas for inflation. This supply can be a container located downhole or a supply line from the surface. In either case, some control mechanism is required to allow inflation and deflation of the bladder for the desired depth of measurement. The bladder serves to reflect rather than dissipate the tube wave energy.
An alternative form of tube wave attenuator is disclosed in S T Chen, "A Single-Well Profiling Tool and Tube Wave Suppression" Expanded Abstract, SEG, 13-16, 1993 and in W T Balogh, "The Borehole Tubewave Damper Probe" Expanded Abstract, SEG, 159-162, 1992, which describe a probe which uses a Helmholtz resonator of high compliance to approximate a high-pass filter of borehole acoustic disturbances which pass the probe. The high compliance results from a gas-filled bladder which fills partially the Helmholtz resonator which comprises a cylindrical steel shell. The bladder is inflated at the surface to a pressure slightly below the ambient hydrostatic pressure in the borehole at the depth of interest and is then lowered into the well for use. This probe is a tube wave reflector and the operating depth depends on the precharge pressure of the bladder.
A still further approach to the problem of tube wave attentuation is proposed in L D Pham, C E Krohn, T J Murray and S T Chen, "A Tube Wave Suppression Device for Cross-Well Applications" Expanded Abstract, SEG, 17-20, 1993. This document describes the use of a cylindrical porous but impermeable body between the source and receivers. Material which is proposed for such use is a material called "syntactic foam". UK Patent Application GB 2253054 describes the use of cork, an absorptive, compressible material, in the form of disks and balls as an attenuator. The effectiveness of both cork and syntactic foam is depth dependent.
It is an object of the invention to provide an effective tube wave attenuator for use in borehole apparatus.